This invention relates to electrochemical apparatus and more particularly to improved devices for electrically measuring the concentration of electrochemically active species in fluids.
The term "fluids" as used herein is intended to include gases, liquids, vapors and mixtures thereof. Heretofore, in the determination of the concentration in fluids of an electrochemically active species (a substance which is capable of being either reduced or oxidized at an electrode), electrochemical devices have been used in which an electrical characteristic of the species is measured and correlated with the concentration. Polarographic devices, for example, have been used measuring the diffusion-limited current at a characteristic potential at which such a species is electroreacted, such current being proportional to the species concentration. An improvement on the basic polarographic apparatus is the well known Clark cell described in U.S. Pat. No. 2,913,386 issued Nov. 17, 1959. The apparatus disclosed in that patent utilizes a dual electrode structure immersed in an electrolyte and encased at least in part in a membrane which is permeable to a predetermined species, for instance, gaseous oxygen. Typically, when used for oxygen analysis the cathode of a Clark apparatus is formed of platinum or gold and is located closely adjacent the membrane; the anode may be formed of silver in some cases and in some structures is made of lead and the electrolyte is usually an aqueous alkali halide solution. In operation such a device when used to measure oxygen has a membrane which is permeable to oxygen so that the oxygen in the fluid being tested, which is outside the membrane, permeates the membrane and is presumably reduced at the cathode to water in accordance with the overall equation EQU 2H.sup.+ + 1/2 O.sub.2 + 2e .fwdarw. H.sub.2 O
it will be apparent that the Clark cell is intended to reduce completely the oxygen permeating the membrane. The current (2e) necessary to effect this reduction is a measurement of the oxygen concentration in the test fluid. In determining oxygen concentration this device typically can employ the silver-silver chloride anode with a potassium chloride electrolyte solution. In that case the anode reaction would be EQU Ag + Cl.sup.- .fwdarw. A.sub.g Cl + e
Where a lead electrode is used for the anode the reaction would be EQU Pb .fwdarw. Pb.sup.++ + 2e
While devices of the type described above, normally referred to as Clark cells, have proved satisfactory for many purposes, several problems may be encountered in certain applications. Some of the disadvantages which are inherent in the Clark type cell arises from the fact that the reactions which give rise to the measured current irreversibly change the internal solution composition. This change may in some cases alter the pH of the electrolyte in time, and result in the physical consumption of the anode. Thus, there may be required either a means for maintaining the pH of the electrolyte as by use of a buffer and/or the supply of sufficient electrode material and electrolyte to withstand the changes without significantly altering the system properties.
Certain of the other problems encountered with the Clark system are more readily understood by reference to FIG. 1, a graphic presentation of the concentration diagram for such devices. In this diagram, the ordinate 60 is the scale from zero of the relative fugacity of oxygen, and the abscissa represents the distance from the cathode-membrane interface toward a test fluid. The position of the cathode-membrane interface (neglecting any small displacement between the two), is at line 60. Line 62, parallel to the ordinate, then represents the membrane-test fluid interface. The distance between lines 60 and 62 is representative of the membrane thickness. Now it may be assumed that the concentration of oxygen in the test fluid adjacent to the outer surface of the membrane is constant, as shown by the horizontal portion of the broken line 64, and that the consumption of oxygen at the cathode is complete so that the concentration of oxygen at the cathode-membrane interface is substantially zero. Under such circumstances the concentration gradient, represented by the remainder of broken line 64, across the membrane is approximately linear and its slope is an inverse function of the membrane thickness.
However, during actual operation of the device, as oxygen is consumed at the cathode, there is a continual flow of the gas from the test fluid through the membrane to replenish the oxygen supply being consumed or reduced. If the oxygen in the test fluid adjacent to the outer surface of the membrane is not continually replenished so as to be maintained at a constant level, the concentration gradient will then extend out into the test fluid, and its slope will become non-linear and reduced as shown at broken line 66 in FIG. 1, due to the local depletion of oxygen in a layer shown between lines 62 and 68. With continued operation of the device and no replenishment of oxygen, the local depletion layer will continue to expand further out into the test fluid, distorting the concentration gradient more and more. The distortion of the concentration gradient reduces the measurement sensitivity and change the mass flow rate of oxygen through the membrane to the cathode, making measurement uncertain and even spurious over a period of time. In order to avoid such occurrence, several means are customarily provided to minimize the extension of the depletion layer into the sample. A minimum fluid flow past the membrane-fluid interface is established, as for instance by stirring, and/or a relatively low permeability membrane may be used perhaps in combination with an inert spacer positioned between the cathode and the membrane thereby increasing the thickness of the electrolyte layer, lessening the flux of the consumed species and minimizing the establishment of the depletion layer in the test fluid.
Further, if the fluid under measurement is a minute sample to which access is restricted, as is frequently the case with clinical samples of biological fluids or cells, depletion will continue until all of the oxygen is consumed. If the consumed oxygen is not or cannot be readily renewed, the measurements in a short time become inconclusive. The input flow rate of the oxygen is often controlled by providing a relatively thick membrane which, however, acts to slow the response time of the device to changes in the oxygen concentration in the test fluid and lessens the magnitude of the signal current.
Further, it is common for the outer surface of the membrane, at line 62, to become fouled to some extent while in use. This problem is particularly acute in applications where the sample is heavily laden with algae, bacterial growth, or particulate. The additional impedance to oxygen flux presented by the fouling causes a diminution of the sensor signal and renders the measurement inconclusive. Wipers to clean the interface, in combination with frequent replacement of the membranes, have been used to minimize this problem.
Some of the above mentioned disadvantages of the Clark type electrode cell are avoided by apparatus of the type described in U.S. Pat. No. 3,260,656 issued to James W. Ross, Jr. on July 12, 1966. The Ross apparatus utilizes a sandwich comprising a cathode and an anode with a spacer between. This sandwich is immersed in an electrolyte and is geometrically oriented so that the electrodes are parallel to a membrane which is permeable to the species being measured. The membrane combines with a housing to enclose the cathode-anode combination in an electrolyte. Typically, as for example, for the measurement of oxygen concentration, the Ross electrode cell utilizes an anode which is formed of a sheet-like element typically having a thickness of about three mils and being porous to both the electrolyte and the electroactive species being measured. The anode is made of an electrically conductive material, preferably a noble metal such as platinum, gold or the like. To provide porosity the anode may be provided as a mesh or screen. The cathode, on the other hand, can be formed of substantially sheet-like material and may be solid and has a thickness which need be determined only by cost and structural strength considerations. The cathode is also preferably made of a noble metal and may be the same metal as the anode. The cathode-anode sandwich is disposed in an electrolyte which is preferably an aqueous solution of a base such as KOH. The spacer between the anode and cathode may, for example, be a sheet-like porous element such as a woven fabric which is electrically non-conductive and chemically inert to the electrolyte.
With the Ross cell, if a potential is applied across the anode and cathode that is well below the decomposition voltage of the electrolyte and if there is no oxygen available in the electrolyte (as from diffusion into the electrolyte through the membrane or from being dissolved in the latter), only a virtually constant, minute, residual current will flow in the cathode-anode circuit. If, however, a supply of oxygen is presented to the outer surface of the membrane, as by contacting the membrane with a liquid having a dissolved oxygen content, or by contacting it with a gas which includes oxygen, then because of the selective permeability of the membrane some oxygen will diffuse through the membrane and thence into the electrolyte to the cathode. If the potential at the cathode is more negative than the reduction potential of oxygen although below the decomposition potential of the electrolyte, oxygen present at the cathode will be reduced. The reduction process is believed to be according to the same equation applicable to the cathodic reduction in the Clark apparatus. With the choice of electrode elements as mentioned above, the anode will cause, by virtue of the anode current, an oxidizing of the water in the electrolyte to generate oxygen according to the following EQU H.sub.2 O .fwdarw. 1/2 O.sub.2 + 2H.sup.+ + 2e
while at the cathode the oxygen reduction occurring is believed to be described by the same equation heretofore used to describe the cathode reduction in the Clark apparatus.
It will then be obvious that the system consumes the species being measured at one electrode such as the cathode and tends to generate a like quantity of that species at the opposite polarity electrode such as the anode, without changing the system such as changing the electrolyte pH and with an appropriate selection of electrode material consumption of the electrode can be avoided and the electrolyte will remain unchanged. The steady-state equality between generation and consumption of oxygen is, however, responsive to any change in concentration of the oxygen outside of the membrane. The gas tensions on both sides of the membrane will tend to reach an equilibrium with one another, thus any change in gas tension outside the membrane will upset the internal steady-state activity of the electrode system forcing it to a new steady-state by either increasing or decreasing the consumption of gas at one electrode with a corresponding increase or decrease in the generation of gas at the other electrode. Each change in gas generation is thus in a direction tending to establish equilibrium between the gas tensions on opposite sides of the membrane and each change in the internal steady-state is accompanied by a change in current flow between the electrodes so that the current flow is generally maintained in direct proportion to the concentration of the oxygen gas, for example, outside the membrane.
Referring to FIG. 2, there is shown graphically a concentration diagram taken across the electrode-membrane structure of the Ross apparatus. As in FIG. 1, the ordinate 70 represents the relative fugacity of the particular electroactive species. The absicissa 72 represents distance from the cathode-electrolyte interface which is at the origin of the graph and hence, in a sense, the latter interface is line 70. Lines 74 and 76 then represent surfaces of an anode positioned between the cathode and membrane, with line 74 representing the surface facing the cathode and line 76 the membrane. Line 78 is the outer surface of the membrane in contact with the test fluid. For the sake of clarity, no displacement between anode and membrane is shown, hence line 76 can also be considered the anode-membrane interface. The displacement between the anode and cathode in the Ross apparatus is maintained by the spacer screen defining the electrolyte layer thickness between lines 70 and 74.
Given the geometry of the Ross apparatus, wherein the generating electrode lies between the consuming electrode and the test fluid, and assuming that the oxygen exhibits the same tension in both the electrolyte and test fluid and the concentration of oxygen in the test fluid is constant, the latter concentration is indicated by the horizontal portion of the broken line 80 which extends from the test fluid into the membrane. The concentration gradient, represented by the remainder of broken line 80, extends presumably from the anode-membrane interface to the cathode-electrolyte interface.
Whereas the Ross cell effectively overcomes the problems of alteration of the electrodes and/or electrolyte, depletion of the oxygen from the test fluid, and extension of the depletion layer into the test fluid causing stirring and fouling dependence, certain other shortcomings are still evident. Among them is the fact that readings with the Ross type cell, obtained by measuring the current flow between the electrodes, tend to stabilize within a maximum of one minute in accordance with the Ross patent. It has been found that response times of this order for continuous measurements, such as the continuous measurement of oxygen concentration, are not suitable for many applications. Increasing the speed of response of the Ross type cell would require decreasing the combined thickness of the electrolyte layer (i.e. the spacer screen thickness) and of the generating electrode, which are already at or near the practical minimum size for these components.
A further disadvantage is the fact that the diffusion layer thickness in the Ross cell is determined by the interelectrode distance, which is subject to variation as the assembly is stressed by forces arising from temperature and/or pressure variations. Not only may the spacing vary, it cannot be less than a few mils to accommodate the spacer screen. Whereas the thickness of the diffusion layer is inversely related to the signal magnitude, an extremely thin and stable diffusion layer is preferred.
A further disadvantage is the cumbersome nature of the layered structure, making reliable fabrication of Ross type devices difficult.
The Ross approach is not the only attempt to overcome the recognized deficiencies of the Clark electrode. Among others, K. H. Mancy used a pulse polarographic technique and J. K. Fowler and K. B. Oldhen used semi-integral amperometry (both reported in "Chemistry and Physics of Aqueous Gas Solutions", The Electrochemical Society, 1975) to minimize Clark electrode shortcomings. However both did so at the expense of complex electronics and at the sacrifice of speed of response, which could exceed one minute.
It is therefore an object of this invention to provide an improved electrode structure which will have all of the advantages of the Clark cell and the improvements of the Ross cell while overcoming their disadvantages. Accordingly, the present invention contemplates an improved electrolytic cell for measuring species concentration in a fluid with increased sensitivity, greater stability and shorter response time when compared to prior art devices such as the Ross cell.